This invention relates to an ultrasonic system for locating the edges of a moving strip web or sheet of material, particularly a moving steel strip, and adjusting the strip center the strip relative to a longitudinal axis.
In steel rolling and sheet finishing mills it is desirable to center a steel strip on a given longitudinal axis and to maintain it on the given longitudinal axis for further processing such as cutting, shaping, galvanizing, annealing, etc. In the past, mechanical sensors have been used to locate and position the strip. These mechanical systems are slow, and in high speed mills, a large section of strip can pass before corrective action can be taken. More recently, optical edge detectors have been used but these are relatively expensive systems which are frequently disabled or rendered less reliable by dirt, oil, moisture, etc. and they usually require a device or surface below or underneath the strip.
The object of the present invention is to provide an inexpensive strip edge sensor, particularly for sensing a moving strip, and still more particularly, a moving steel strip and which needs nothing underneath the strip and which operates in difficult ambient environments such as dust, moisture, oil drips, etc. A further object of the invention is to provide an edge sensing system for steel strip and for detecting changes in the position of said edges and maintaining the center of the moving steel strip on a predetermined longitudinal axis.
According to the invention, a pair of ultrasonic transducers is mounted above the center of the path of the moving strip. The transducers are oriented at an acute angle .varies. relative to the plane of the sheet so that, while ultrasonic reflections from the surface are away from the transducer, the edges are an acoustic discontinuity so that an acoustic pulse in a beam insonifies the edge and, hence, provide an acoustic diffraction which can be detected by the transducers to thereby locate the edges. The speed of sound in the medium is not a factor involved in the centering function. If the speed of sound is known, an ultrasonic height measurement can be made and using the signal times of the edges, a computation of the width of the moving sheet made therefrom. Moreover, when used in a steel sheet processing mill where there is dust, dirt, moisture, oil drippings, etc., the system can be used advantageously to maintain the position of the strip centered along a given axis. The system will produce high accuracy on any smooth sheet. Degradation will occur when roughness dimensions of the sheet become appreciable compared to the wavelength of the ultrasonic signal. With a rough surface, energy will be back-scattered from the surface to the transducer and will interfere with reception of the diffraction signal with resultant timing error.